Field of the Invention
Embodiments of the invention relate to an electrically conductive resin composition and a film thereof. More specifically, embodiments of the invention relate to an electrically conductive resin composition that enables to form a film suitable for an electrode in a storage battery and a protective coating on the electrode, and a film of the electrically conductive resin composition.
Description of the Related Art
Recently, so-called renewable energy, such as solar power, wind power and wave power has received attention as a new energy source alternative to fossil fuel such as petroleum and nuclear energy. However, the output of these renewable energies is extremely unstable due to being highly susceptible to weather or the like. Accordingly, in order to send these energies to a power network in a large amount, it is necessary to level the variation of output by equipping with a large-capacity storage battery, for example.
One example of such large-capacity storage batteries is a redox flow battery. A redox flow battery contains two types of ionic solutions separated by a cation-exchange membrane, and operates with charges/discharges by allowing oxidation reaction and reduction reaction to proceed simultaneously on electrodes provided in the respective solutions. For example, in the case of a redox flow battery using an aqueous solution of vanadium in sulfuric acid for both electrodes, when charged a tetravalent vanadium is oxidized to a pentavalent one on the anode, and a trivalent vanadium is reduced to divalent one on the cathode. When discharged, the reverse reactions occur. A redox flow battery has an advantage of easiness in increasing a facility in size. In addition, a redox flow battery operates at a room temperature and uses no substance having combustibility or explosiveness, and farther does not cause the generation of such a substance. Therefore, a redox flow battery is excellent in safety as compared with a sodium-sulfur battery and a lithium-ion secondary battery.
Electrodes in a redox flow battery are soaked in an electrolytic solution such as an aqueous solution of sulfuric acid and redox reaction occurs thereon. Therefore, the electrodes need a high conductivity and chemical resistance and a carbon fiber aggregate or a platinum plating is used for the electrode. However, a carbon fiber aggregate has liquid permeability, and has a disadvantage that a connection part of a carbon fiber aggregate and a copper wire is eroded by a transported aqueous solution of sulfuric acid or the like. Meanwhile, a platinum plating is a very good conductor and excellent in chemical resistance, but has a disadvantage of being expensive because of being a noble metal.
In view of this, an electrically conductive resin film in which an electrically conductive carbon, such as Ketjen black, is kneaded has been used for a conventional electrode, or an electrode of a carbon fiber aggregate or a copper sheet has been coated with the electrically conductive resin film. However, these electrically conductive resin films have a disadvantage that a large amount of an electrically conductive carbon kneaded therein for the purpose of imparting a sufficiently high conductivity makes the electrically conductive resin film very insufficient in tensile elongation, bending resistance and flexibility and fragile to physical force. Meanwhile, in the case that the amount of an electrically conductive carbon to be formulated is reduced to ensure the tensile elongation, the bending resistance and the flexibility, the volume resistivity becomes over 10 Ω·cm. A redox flow battery using such an electrically conductive film as an electrode or a coating thereon is not satisfactory in that the internal resistance is increased.
On the other hand, recently, a carbon nanotube has received attention as an electrically conductive carbon and expected to solve the above disadvantages. However, it is difficult to defibrate a carbon nanotube, and therefore a carbon nanotube has a disadvantage of being very difficult to be dispersed in a resin. Due to this, in order to obtain a sufficiently high conductivity, a large quantity of carbon nanotubes must be formulated as with Ketjen black. As a result, the large quantity of carbon nanotubes formulated makes the tensile elongation, the bending resistance and the flexibility of the electrically conductive resin film insufficient in practical use. Further, when a higher shear stress is applied in a step of defibrating/dispersing for the purpose of improving the defibrated/dispersed state of a carbon nanotube, the carbon nanotube will be broken. Thus, even if a higher shear stress is applied in a step of defibrating/dispersing, it is still necessary to formulate a large quantity of carbon nanotubes in order to obtain a sufficiently high conductivity.
An electrically conductive film comprised of a composition, which is obtained by mixing a carbon black or a carbon nanotube with a propylene-olefin copolymer wax to make a master batch and mixing this with an organic polymer, also has been proposed in the conventional art. Although this master batch allows a large quantity of a carbon black or a carbon nanotube to be filled, the film to be obtained tends to have an insufficient conductivity.